1. Field of the Invention
The present invention relates to a system for controlling a vehicular motion and, more particularly, to a vehicular motion controlling system for controlling the vehicular motion when an engine brake is applied to a vehicle or when a vehicle is in a turning state.
2. Related Art
In the prior art, when the engine brake is applied by releasing the throttle valve on a low-friction road such as a frozen road, the rotational speed of the drive wheels drops with respect to the vehicular body speed, and as a result, the drive wheels may slip. When the drive wheels slip, there arises a phenomenon that the lateral resistant force of the drive wheels falls.
When the slip is increased by the engine brake at a time of turning state of the vehicle, for example, if the lateral resistant force of the drive wheels is reduced, as illustrated in FIG. 1A, a turning moment occurs in an FR car, and the vehicle may become unstable to cause the tuck-in or the spin. On the other hand, an FF car may become unsteerable although the vehicle will not spin.
In order to prevent this, there has been proposed a technique (as disclosed in Japanese Patent Laid-Open No. 62-153533), in which an engine output is raised on the basis of the output state of the engine and the lateral acceleration in order to reduce the engine braking force so that the slip ratio of the wheels may fall within a predetermine range.
In order to prevent the spin or the tuck-in at the time of engine braking, however, the engine output is raised in the above-specified control, and there arises another problem that the deceleration, as expected by the driver, cannot be realized.
In the aforementioned prior art, the engine output torque is not raised before the slip state appears in the drive wheels. Because of the delay due to the inertia of the mechanism in the engine and the delay due to the inertia of a drive system from the engine to the drive wheels, the timing for control to increase the rotational speed of the drive wheels actually may be delayed. As a result, the drive wheels are locked to raise a problem that the running stability is deteriorated.
As a technique for enhancing the steering stability at the time of a turning state of the vehicle, on the other hand, there is known yawing rate control.
In this yawing rate control, a target yawing rate is determined from the steering operation of the driver and the like to control the torque distribution ratio to the left and right drive wheels or to control the engine output for imparting the drive torque to the left and right drive wheels so that the real yawing rate of the vehicle approaches the target yawing rate. As disclosed in Japanese Patent Laid-Open No. 7-164924, for example, during an under-steering state in which the target yawing rate is higher than the rear yawing rate, the engine output is raised, and the distribution of transmitted torque to the outer wheel on a turning circle between the left and right drive wheels is increased to enlarge the torque difference between the left and right drive wheels. During an over-steering state in which the target yawing rate is lower than the real yawing rate, on the other hand, the engine output is lowered to bring the drive wheels into the engine braking state thereby to reduce the torque difference between the left and right drive wheels.
In this yawing rate control of the prior art, however, the real yawing rate is controlled to the target yawing rate merely by the torque difference of the left and right drive wheels, and as a result, the stability of the vehicle may fail to be retained.
In the rear wheel drive car, for example, when the driver steers sequentially leftward and rightward to change the lane or to run on an S-curve, the torque difference of the drive wheels is increased according to the aforementioned yawing rate control in response to the first (or leftward) steering operation, to establish the turning moment in the vehicle. Accordingly, the torque difference of the drive wheels is increased in the opposite direction in response to the next (rightward) steering operation. At the yawing rate control accompanying this second steering operation, however, the vehicle exhibits an extremely high under-steer tendency, whereby the torque difference of the drive wheels is enlarged extremely. As a result, when the steering is returned straight, even though the torque difference of the drive wheels is reduced to zero, but this reduction may not be effected in time to prevent the vehicle from spinning.
Especially when the vehicle runs on a low-.mu. road having a surface of low coefficient .mu. of friction, the reaction (i.e., the road surface reaction) for the wheels to receive from the road surface is so small that the yawing moment to be generated by the drive torque difference is reduced. Therefore, when the yawing moment for turning round the vehicle quickly is given during the run on the low-.mu. road in response to the steering operation of the driver, the yawing moment toward the opposite direction, as required by the steering operation in the opposite direction, cannot be generated. As a result, the vehicle comes into an extremely unstable state to become liable to spin.
In other words, yawing rate control of the prior art can enhance the steerability but not retain the stability of the vehicle.